1. Field of the Invention
This invention concerns videocassette tape spools, and particularly those which include abrasion-resistant wear buttons that ride against leaf springs mounted on the upper, inner faces of the cassettes.
2. Description of the Prior Art
Each of the Betamax, VHS and compact VHS-C videocassettes has a pair of tape spools loosely positioned side-by-side within the cassette housing. When the cassette is inserted into a videotape recorder, a pair of spindles fit into interior teeth of the hubs of the spools and press the spools against a leaf spring which is mounted to the inner, upper face of the cassette. The spring has two arms, each of which contacts an abrasion-resistant wear button on each of the tape spools.
The tape spools of a typical videocassette has two parts, each a single piece of molded plastic. The first piece is opaque and includes a hub and one flange. The second piece is a transparent plastic disk which provides the other flange and has a central opening through which the wear button protrudes.
The typical prior practice is typified by U.S. Pat. No. 4,289,282 and Japanese utility model No. JA54-37123, wherein the wear button is molded as an integral part of the hub and projects through a hole provided in the transparent flange. Since the wear button must be of an abrasion-resistant plastic such as acetal resin, this integral molding technique requires that the entire hub and its attached flange be likewise molded of this material. Since acetal resin is relatively expensive, this design is economically disadvantageous.
In an effort to reduce costs, the wear button was subsequently molded as a separate piece which could be assembled to the integral hub and flange and captured by the clear closing flange when the spool was assembled. Such a construction is illustrated by Japanese utility models Nos. JA54-70123 and JA55-158080 and German Pat. No. DE 3222632 Al. European Pat. No. 0 072 517 A3 discloses a steel ball which rests in a socket formed in the hub to provide an abrasion-resistant wear button.
These designs are an improvement over the integrally molded concept in that the wear button may consist of a relatively expensive material while the hub and flange to which the wear button is attached may be molded from a cheaper material such as acrylonitrile-butadiene-styrene (ABS) or high-impact polystyrene (HIPS). However, it has been found that the loose capture of the wear button between the hub and flange or the positioning of a spherical wear button within a socket allows the wear button to rotate relative to the hub or the socket. This relative rotation of the abrasion-resistant wear button relative to the softer material comprising the mounting portion of the hub causes the softer material of the hub to melt or abrade, and the wear button may actually melt or abrade its way completely through the material from which the hub is molded.
It is necessary, therefore, that the abrasion-resistant wear button be prevented from rotating relative to the hub upon which the wear button is mounted and it is highly desirable that the mounting arrangement between the wear button and the hub be common for each of the Betamax, VHS and VHS-C videocassette spools so that a single, common wear button may be used with each of these slightly different constructions.